Synaptic innervation of neurons in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys

Abstract

In order to better understand the way by which the subthalamic nucleus interacts with the globus pallidus to control the output of the basal ganglia, we carried out a series of experiments to investigate the pattern of synaptic innervation of the pallidal neurons by the subthalamic terminals in the squirrel monkey. To address this problem we used the anterograde transport of biocytin. Following injections of biocytin in the subthalamic nucleus, rich plexuses of labelled fibres and varicosities formed bands that lay along the medullary lamina in both segments of the ipsilateral pallidum. At the electron microscopic level, two populations of bioctyin-containing terminals were identified in the internal pallidum (GPi). A first group of small to medium-sized terminals (type 1) mean cross-sectional area ±S. D. = 0.41 ± 0.04 μm² contained round vesicles and formed asymmetric synapses with dendritic shafts (95%) of mixed sizes (maximum diameter ranging from 0.3 to 4.0 μm) and spine-like structures (5%). This second group of terminals (type 2) contained pleiomorphic vesicles, had a larger cross-sectional area (mean ± S. D. = 0.9 ± 0.4 μm²) and formed symmetric synapses predominantly with perikarya (41%) and large dendrites (57%). In some cases, the two types of terminals converged at the level of single GPi neurons. Postembedding immunogold method revealed that the type 2 terminals displayed gamma-aminobutyric acid (GABA) immunoreactivity, whereas the type 1 terminals did not. In the external pallidum (GPe), injections in the subthalamic nucleus labelled both type 1 or type 2 terminals. However, the labelled type 2 boutons were much less abundant in GPe than in GPi. The presence of biocytin-labelled perikarya in GPe and the fact that the type 2 terminals displayed GABA immunoreactivity led us to suspect that these terminals were derived from axons of GPe neurons. In agreement with this hypothesis, injctions of Phaseolus vulgaris-leucoagglutinin (PHA-L) in GPe labelled terminals in GPi that displayed the morphological features and a pattern of synaptic organization similar to the type 2 terminals. In conclusion, the results of our study demonstrate that the subthalamopallidal terminals form asymmetric synapses that are distributed along the dendritic tree of GPe and Gpi neurons. In contrast, the GPe projection to GPi give rise to large GABA-containing terminals that form symmetric synapses predominantly with the proximal region of pallidal neurons. Because the GABAergic axon terminals from GPe form synapses onto the perikarya and proximal dendrites of GPi neurons, the Gpe input is in a strategic position to reduce the excitatory influence generated more distally on the dendritic tree by the subthalamic nucleus.